Critical Care Trauma Centre

A "bypass" is an operation that is performed in order to "detour" blood around an area of blood vessel narrowing. Bypass operations can be performed in different areas of the body, but the most common type of bypass procedure is performed on the arteries of the heart (calledCoronary Arteries). Bypasses are also frequently performed on the blood vessels of the legs if atherosclerotic disease is causing a reduction in blood flow to the lower extremities.

The coronary arteries are the arteries that supply the heart muscle with oxygen. They run along the outside of the heart muscle (Image 1). If they become narrowed, the heart muscle may not get as much oxygen as it needs. Diseased vessels are also prone to clot formation. Blood clots that develop can obstruct the flow through a vessel and cause an area of the heart muscle to die from lack of oxygen. This is the most common cause for a " heart attack", called a "Myocardial Infarction (MI)".

A Coronary Artery Bypass Graft (also called CABG, pronounced "Cabbage") is a procedure that uses veins and or arteries from other areas of the body to create new "routes" for blood to reach the heart muscle. The new pathways are called "grafts" because they are "grafted" or sewn to the diseased vessel.

Usually, heart surgery is performed through an open incision in the chest (called "Open- Heart Surgery"). When the open-heart approach is used, the heart and lungs are usually stopped during the procedure. In order to ensure that the body tissues continue to receive oxygen during the procedure, the patient is connect to a Heart-Lung Bypass Pump (often called "Pump" or Bypass Pump). The Heart-Lung Bypass pump will temporarily perform the functions of the heart and lung. In London, Heart Surgery is done at University Hospital.

During Heart-Lung Bypass, large catheters are inserted into the right side of the heart and connected to the Heart-Lung Bypass Machine. A return catheter is inserted into the aorta ( the large blood vessel that normally receives all of the blood as it leaves the left side of the heart). The "pump" pulls blood from the right-sided catheters and returns it to the aorta, "bypassing" the heart and lung. The pump keeps the blood circulating through the remaining arteries and veins, temporarily replacing the pumping action of the heart. Blood is pulled through a special filter that introduces oxygen and removes carbon dioxide before it is returned to the aorta. This filter temporarily replaces the function of the lung and ensures that the circulating blood contains an adequate supply of oxygen. .

The term "Bypass" can be confusing, because it is often used in heart-surgery with two different meanings. While a Bypass procedure (such as a coronary artery bypass) creates new pathways for blood to flow, the patient is usually connected to a "Bypass Machine" during the surgery. While the most common type of Open-Heart Surgery requiring the use of a Heart-Lung Bypass Machine is Coronary Artery Bypass surgery, the Heart-Lung Bypass machine is used for many other heart surgeries including heart valve replacement or repair of birth defects of the heart (called congenital defects). On rare occasions, the Heart-Lung Bypass machine can be used to rapidly rewarm and oxygenate a patient with severe hypothermia (for example following prolonged immersion in cold water).

One method for creating a Coronary Artery Bypass Graft is to take a piece of "donor vein or artery", and connect one end to the aorta and the second end below the narrowed vessel (Image 2). Blood will then flow through the bypass vessel to carry oxygenated blood to the area below the narrowing.

Another common method is to use an existing artery that normally provides blood flow to areas near the heart. The artery is disconnected and reattached to the diseased coronary artery. The vessel that is used most frequently is an artery that supplies the chest wall, called the Internal Thoracic Artery. The more common name for this vessel is the Internal Mammary Artery (or "IMA"). If the left IMA is used, the graft is often called a "LIMA" (Left Internal Mammary Artery) [Image 3]. A RIMA (Right Internal Mammary Artery) graft can also be performed. When one (or occasionally both) arteries are used, other blood vessels in the area will usually be able to meet the blood flow needs of the chest wall.

Image 1: The coronary arteries are located around the outside surface of the heart. They carry oxygenated blood to the heart muscle.

Image 2: A blood vessel attached to the aorta carries blood to the coronary artery below an area of blockage. This new artery "bypasses the narrowed area".

Although arterial grafts have a longer life expectancy than vein grafts, there are a limited number of suitable vessels. A combination of arterial and vein grafting is often used during a procedure in order to adequately bypass diseased vessels.

Image 3: An example of a Left IMA graft (using the Left Internal Mammary Artery).

Image 4: A Heart-Lung Bypass Pump. Blood is pulled from the patient before it reaches the heart and lung. The pump pulls the blood through the special filter, where the blood is reoxygenated. The purified blood is pumped back into the patients blood stream beyond the heart, "bypassing" the heart and lung.